Polyolefin compositions having elastic properties while maintaining a good thermoplastic behavior have been used in many application fields, due to the valued properties which are typical of polyolefins, such as chemical inertia, mechanical properties and nontoxicity. Moreover, they can be advantageously transformed into finished products with the same techniques used for thermoplastic polymers. In particular, flexible polymer materials are widely used in the medical field, as well as for packaging, extrusion coating and electrical wires and cables covering. In many of these applications, vinyl chloride polymers containing adequate plasticizers, which are necessary to give said polymers their desired flexibility characteristics, are presently used. Said polymer products, however, are subject to ever increasing criticism both for the suspected toxicity of the plasticizers they contain and because when incinerated, they can disperse into the atmosphere extremely toxic by-products, such as dioxin. It would be very useful, therefore, to substitute said materials with products which besides the desired flexibility characteristics and transparency, would have the chemical inertia and nontoxicity typical of olefin polymers. Elastic polypropylene compositions retaining good thermoplastic behavior have been obtained in the art by way of sequential copolymerization of propylene, optionally containing minor quantities of olefin comonomers, and then ethylene/propylene or ethylene/alpha-olefin mixtures. Catalysts based on halogenated titanium compounds supported on magnesium chloride are commonly used for this purpose. For instance, EP-A-472 946 describes flexible elastoplastic polyolefin compositions comprising, in parts by weight: A) 10-50 parts of an isotactic propylene homopolymer or copolymer; B) 5-20 parts of an ethylene copolymer, insoluble in xylene at room temperature; and C) 40-80 parts of an ethylene/propylene copolymer containing less than 40% by weight of ethylene and being soluble in xylene at room temperature; the intrinsic viscosity of said copolymer is preferably from 1.7 to 3 dl/g. Said compositions are relatively flexible and have good elastic properties, as demonstrated by flexural modulus lower than 150 MPa values, Shore D hardness from 20 to 35, and Shore A hardness of about 90, associated with good tension set values (of 20-50% at 75% elongation, and about 33-40% at 100% elongation); nevertheless, such values are not fully satisfactory for many applications. Mineral fillers, such as aluminum and magnesium hydroxides or calcium carbonate, are commonly used at high concentration levels in polyolefin compositions for several reasons, for instance to impart self-extinguishing properties or to improve application-related physical properties, such as soft touch and printability. The major disadvantage of these mineral fillers, in particular when used on functional grounds as in the case of flame retardants, is the very high loading needed. Depending on the class of fire-retardancy requested, up to 65-70% by weight of filler can be necessary in order to reach adequate effectiveness in polyolefins: A lower amount of filler around 40-60% wt can be also sufficient for flame retardancy in certain applications. Normally, this has a highly negative influence on the processing of the polymer, with difficulties in adding and dispersing such high levels of filler, and on the physical-mechanical properties of compounds, namely lower elongation at break, lower tensile strength and higher brittleness. EP 1 043 733 describes self-extinguishing electrical cables having a coating layer based on a polymer material containing a flame-retardant inorganic filler; this polymer material comprises a heterophase copolymer having at least 45% by weight of an elastomeric phase based on ethylene copolymerized with an alpha-olefin, and a thermoplastic phase based on propylene. While these compositions incorporate large amounts of flame-retardant filler, the very high levels of filler negatively affect the physical-mechanical properties of the polymer material, and in particular lead to low elongation values. As a result, the final product is no longer apt to various applications, such as roofing, membranes and cables. In order to compete with plasticised PVC in the above applications, it would be necessary to provide flexible polyolefin compositions, having low flexural modulus and hardness values, capable of incorporating large amounts of filler without deterioration of physical and mechanical properties, and in particular elongation at break, stress at break, tension set and improved behaviour at low temperature.
More flexible elastoplastic polyolefin compositions have been described in the International Application WO03/011962, and comprise, by weight:
A) 8 to 25% of a crystalline polymer fraction selected from propylene homopolymer and propylene copolymers with a C4-8 alpha-olefin;
B) 75 to 92% of an elastomeric fraction comprising two different propylene elastomeric copolymers, and more specifically: (1) a first elastomeric copolymer of propylene with 15 to 32% of ethylene and or a C4-8 alpha-olefin, and (2) a second elastomeric copolymer of propylene with more than 32% up to 45% of ethylene and/or a C4-8 alpha-olefin, the (1)/(2) weight ratio ranging from 1:5 to 5:1. These polyolefin compositions have flexural modulus lower than 60 MPa, Shore A lower than 90, and tension set at 100% elongation lower than 35%. The compositions described in this document do not contain relevant amounts of fillers.
In the International Application WO2004/026957 the flexible polyolefin compositions described in WO03/011962 are filled with 40 to 80% by weight of an inorganic filler, selected from flame-retardant inorganic fillers and inorganic oxides or salts, without loosing their physical-mechanical properties, and in particular retaining low hardness and flexural modulus values, high elongation at break and low tension set values. and The highly filled soft polyolefin compositions described in WO2004/026957 have preferably Shore A hardness lower than 90, elongation at break (ASTM D638) higher than 400%, tensile strength at break (ASTM D638) equal to or higher than 4 MPa.